Classes in named Java 9 modules can't be instrumented

[fmeum]

Instrumenting internal classes outside the default module (e.g., java.desktop) is tricky as of Java 9: As the agent runs in a context that is not module-aware, it can't export CoverageMap to other modules. This leads to NoClassDefFound exceptions when classes from other modules are instrumented.

In order to work around this, we may have to split the runtime package off and turn it into a proper Java 9 module.

Originally posted by @fmeum in #98 (comment)